1. Field of the Invention
The present invention generally relates to a computerized control system for the drive motor of a dental handpiece and, more particularly, to such a control system for taking account of the condition of cutting tools used in the handpiece.
2. Description of the Related Art
Dental handpieces are used in root canal work for motorized driving of root canal files and other cutting implements. The breaking of a root canal file is a serious event, particularly where the broken end of the file may remain embedded in the root canal of the tooth. File breakage, as currently understood, occurs when torsional stress thresholds for a given instrument are exceeded during its rotation in the tooth. Also contributing to this instrument failure is the accumulation of cyclic fatigue as this instrument rotates in root canals that are curved. Such cyclic fatigue develops due to the tensile stretching that occurs in this file on the outside of its curvature and the compression that occurs on the inside curvature at that same location along the file's length. This compression/tension event is magnified by the rapid rotation of the instrument during root canal preparation procedures, alternately stretching and compressing the file around its circumference.
Numerous efforts have been made by innovators in the field to limit file breakage primarily by measuring torque values imparted to the file during root canal shaping procedures and to take corrective steps, such as sounding an audible warning and/or slowing, stopping or reversing the handpiece motor when a reference level is reached. However, this torque limitation safety feature does not take account of accumulated fatigue which may contribute to a reduction of permissible torque limit for a given instrument, or the curvature of the root canal being shaped, thus resulting in actual breakage. Neither does it account for the variations in structural characteristics of the different shaping files used to prepare a root canal.
A number of examples of such efforts from the prior art are set out in the following.
U.S. Pat. No. 4,243,388 of Arai discloses a dental hand engine for driving a reamer for root canals. The disclosed system incorporates a control device which electrically senses when the forward end of the reamer reaches the radical apex of a tooth root canal so that the engine and its reamer may be automatically stopped.
U.S. Pat. No. 4,955,810 of Levy discloses apparatus and method for measuring thickness of the dentin layer of a patient's tooth. In the disclosed process, the resistance to electrical current flow from a voltage applied between the dentin and a spaced apart region of the patient's body is used to indicate the thickness of the dentin layer. U.S. Pat. No. 5,538,423 of Coss et al. discloses a dental drilling system having a programmable control unit for controlling operating parameters of the drilling system, such as direction of rotation, speed of rotation, torque of the dental drill tool bit, pumped irrigation fluid flow rate, and the intensity of light from a light source, to name a few. The control unit may be programmed with sets of data values representing a desired value for each of the operating parameters to be controlled. The system is said to accurately achieve and maintain a specified rotation speed or torque.
U.S. Pat. No. 4,723,911 of Kurtz discloses apparatus for high-speed drilling of bone tissue of varying density to produce proportioned variations in the speed of the drill. Instantaneous bur rotational speed is automatically sensed to produce a signal representing a change in speed, correlating to an indication of the density of the bone tissue.
A number of systems for controlling motor speed or other parameters of a drill system outside the field of endodontics are known. For example, U.S. Pat. No. 5,038,084 of Wing describes a closed loop control system which senses the current of a drill motor in order to cause the drill motor to slow down as current decreases, such as when the drill cuts through a workpiece.
U.S. Pat. No. 5,543,695 of Culp et al. discloses a powered medical instrument including a manually operable foot switch for a motor control unit coupled to an autoclavable handpiece. In operation, a maximum torque value is set for a control circuit which limits the motor torque to the selected value.
U.S. Pat. No. 5,116,168 of Aihara discloses a control system for a machine drill making holes through a composite workpiece made of materials having different machining properties. Sensors detect feed speed and rotational speed, respectively, as well as thrust force applied to the drill. The resulting sensor signals are compared with optimum values to control the feed speed and rotational speed of the drill.
U.S. Pat. No. 4,822,215 of Alexander discloses an automatic drill system which also utilizes thrust and torque sensors for enabling a computer to control the drill for an efficient drilling operation for laminated materials, a non-dental application.
U.S. Pat. No. 5,980,248 of Kusakabe et al. discloses a motor controller for a dental handpiece which uses signals from a torque sensor which detects the load torque applied to the cutting tool to either stop the drive motor, reduce the rotational speed thereof or temporarily reverse motor direction when the detected load torque has reached a preset reference value. This system is said to prevent a cutting tool, such as a relatively slender file for root canal formation, from breaking during the procedure.
The breaking of a root canal file with the broken-off part remaining in the tooth undergoing treatment represents somewhat of a disaster. Any system which relies on the sensing of drive motor torque or file tip position and comparing the read-out signals with preset reference levels is still subject to failure (i.e., file breakage) when the reference levels are not set properly.
Unexpected file breakage in a system in which the torque or other parameter sensors are functioning properly and the preset reference levels are within normal limits can still occur in instances where the file itself may have developed fatigue from repeated use in the standard endodontic procedure so that its likelihood of breakage is no longer governed by the statistical data applicable to a standard population of unused files.
Unexpected file breakage can also be caused by obvious and hidden canal curvatures. Canal curvatures which introduce fatigue to files range from even slight coronal curvatures to severe apical curvatures. Furthermore, molar root canal curvatures are always multiple in number and multiplanar in direction. Endodontic shaping files are subject to cyclic fatigue when curvatures are significant, even when the canal is relatively large and little torsional stress is imparted to its fatigue history. Even the largest file diameters will break when challenged by cyclic fatigue.
Fatigue in any drilling implement is difficult to determine. Therefore it is difficult to predict whether a given drill will perform satisfactorily in a control system where the reference limit values are standard and where the sensors for measuring torque or other parameters are performing properly.